Habitat preferences of Aporosa in two Malaysian forests: Implications for abundance and coexistence

Theories accounting for the maintenance of high tree diversity in tropical rain forests range from those proposing that tropical trees are highly co-evolved niche specialists, to those proposing that they are mostly generalist, undergoing random drift. We test these hypotheses at a meaningful, community-wide scale using data on the spatial patterns and habitat preferences of all species of Aporosa (Euphorbiaceae) growing on two large rain forest plots in Malaysia. Second-order spatial pattern analyses using a method based on Ripley's K function showed that Aporosa species formed spatially distinct assemblages, and a randomization procedure suggested that these assemblages were explained by biases in their distributions in relation to habitat types. Soil type, as determined by parent material, was an important determinant of habitat preferences, although topography and forest structure also accounted for some variation. We conclude that niche differentiation is an important mechanism contributing to the coexistence of Aporosa species at the community scale. However, spatial separation due to these differential habitat biases accounted for only a portion of the high species richness observed in this genus, so other mechanisms must also be sought to account fully for the maintenance of tropical tree species richness.